/*
 * Copyright (C) 2011 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.gwt.appengine.channel.client;

import com.google.gwt.dom.client.Document;
import com.google.gwt.dom.client.ScriptElement;
import com.google.gwt.user.client.Timer;

/** Manages creating {@link Channel}s to receive messages from the server. */
public class ChannelFactory {

  private static final String CHANNEL_SRC = "/_ah/channel/jsapi";
  private static Channel channel;

  /**
   * Creates a {@link Channel} with the given client ID, passing it to the given
   * {@link ChannelCreatedCallback}.
   *
   * <p>
   * If a Channel has already been created, this will immediately be passed to
   * the callback, so only one Channel will ever be created by this method.
   * </p>
   */
  public static void createChannel(final String clientId, final ChannelCreatedCallback callback) {
    if (channel == null) {
      ScriptElement script = Document.get().createScriptElement();
      script.setSrc(CHANNEL_SRC);
      Document.get().getElementsByTagName("head").getItem(0).appendChild(script);

      new Timer() {
        @Override
        public void run() {
          if (scriptLoaded()) {
            channel = createChannelImpl(clientId);
            callback.onChannelCreated(channel);
            this.cancel();
          }
        }
      }.scheduleRepeating(100);
    } else {
      callback.onChannelCreated(channel);
    }
  }

  private static native boolean scriptLoaded() /*-{
    return !!$wnd.goog && !!$wnd.goog.appengine && !!$wnd.goog.appengine.Channel;
  }-*/;

  private static final native Channel createChannelImpl(String clientId) /*-{
    return new $wnd.goog.appengine.Channel(clientId);
  }-*/;

  public interface ChannelCreatedCallback {
    public void onChannelCreated(Channel channel);
  }
}
